Wafer prealigner using pulsed vacuum spinners

ABSTRACT

Wafer alignment apparatus includes an aligner platform having defined thereon X and Y axes and a desired rotational orientation for a wafer. A first vacuum spinner is supported for rotation in the platform about the intersection of the X and Y axes and second and third spinners are provided displaced from the intersection of the axes. Two edge sensors, for sensing the desired wafer edge location at two points separated from each other, and edge or notch sensors for sensing rotational alignment of said wafer provide inputs to logic means which provide outputs to the vacuum spinners for carrying out a series of individual rotations to align the wafer.

BACKGROUND OF THE INVENTION

The invention relates to micro-circuits, in general, and moreparticularly to an improved wafer prealignment system for use in theprocessing of wafers of silicon and the like to make micro-circuits.

As it is well known by those skilled in the art, for the making ofmicro-circuits, a wafer of silicon, which may be as large as 120 mm indiameter, for example, must be processed through a number of steps.Included among these steps are steps of dry processing (e.g., anisotopic etching) and steps of fine alignment for exposure purposes. Inthe case of dry processing, absolute alignment accuracy is notessential. In the case of exposing the wafer, accuracy beyond thatobtainable with a prealignment system is necessary. For this purpose,the machines which are used for exposing wafers generally have finealignment systems. However, for the fine alignment systems to beeffective, a prealignment within a certain tolerance, for example,within plus or minus 0.25 mm is necessary to avoid an essentially randomsearch in the fine alignment system. The smaller the prealignment errorthe better. In other words, with better prealignment, a simpler andfaster fine alignment system becomes possible.

Various prior art prealignment systems have been developed. Typically,these handle the wafers by the edges. Handling by the edges isundersirable since it can result in contamination or breakage. Systemswhich do not touch the edges have been developed. However, they cannotprovide the necessary speed, accuracy, and reliability of alignment.

Thus, it is the object of the present invention to provide an improvedprealigner and a method of carrying out prealignment which quickly andeffectively gives the required accuracy for dry processing and forprealignment in a device where further fine alignment takes place, whichdoes not require handling the edges of the wafer.

SUMMARY OF THE INVENTION

The present invention accomplishes this object by the use of vacuumspinners. Each vacuum spinner utilized is capable of grabbing the backof the wafer and rotating. The vacuum spinners include an on-axis and atleast one off-axis spinner. The spinners are operatively coupled to thewafer in sequence under the control of a logic system which may beimplemented in a hard-wired form or with a micro-processor or othercomputer. Inputs to the logic system for optical or pneumatic sensorsindicate the edge positions and control the sequence of pulsing, and thedirection of rotation of the spinners.

In the preferred embodiment, a center vacuum spinner, which is on-axis,and left and right vacuum spinners are utilized. Associated with thecenter of each of the left and right spinners is an optical or pneumaticsensor. In addition, three sensors in a row are provided to detect aflat or notch in the wafer to insure proper rotational alignment. In theillustrated embodiment, control is carried out by means of a programmedmicro-processor. The system illustrated carries out sufficientinteraction to align the wafer for dry processing. If a more accuratealignment is required, additional iterations can be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the system of the present invention.

FIG. 2 is a plan view of the arrangement of the vacuum spinners of thepresent invention showing the location of the spinners and the sensors.

FIG. 3 is a cross section through a vacuum spinner.

FIG. 4 shows how FIGS. 4A and 4B are to be combined.

FIGS. 4A and 4B are a flow diagram for the micro-processor of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of the prealignment system of the presentinvention. The prealigner itself is indicated by block 11. Theprealigner receives inputs from and provides outputs to an I/O port 13associated with a micro-processor 15. Micro-processor 15 receives aninput on a line 17 from the system with which it is used, enablingprealignment. Microprocessor 15 is coupled to I/O port 13 over a bus 19in conventional fashion. Within the prealigner are three vacuumspinners, a center vacuum spinner 21, a right vacuum spinner 23 and aleft vacuum spinner 25. Each of these contains a plurality of outlets 27which can be coupled to a vacuum source to grab an overlying wafer in amanner to be seen more fully below. Associated with the right spinner 23is a sensor 29 and with the left spinner a sensor 31. Outputs from thesesensors are coupled into the I/O port 13. In addition, left, middle andright optical flat detector 33-35 provide outputs to the I/O port 13.Based on these outpts, in accordance with a program to be described morefully below, the I/O port 13 provides outputs to vacuum line solenoidvalves 37, 39 and 41 associated; respectively, with the spinners 21, 23and 25. Each of the solenoid valves 37, 39 and 41 receives an input froma vacuum source on line 43 and, when operated by its associated controlline, couples that vacuum to its associated spinner.

The operation of the vacuum spinners can better be seen from FIG. 2,which is a schematic plan view of the arrangement of the spinners on analigner platform 45. Each of the spinners 21, 23 and 25 is supported forrotation in an appropriate bore in the aligner platform 45. Alsoprovided in the aligner platform 45 is a suitable recess for transferfingers 47 having on at their end vacuum ports 49 for grabbing the waferonce it is prealigned to the transfer it, in the prealigned condition,to a location where additional alignment or dry processing will takeplace. Between the transfer fingers 47 are the flat or notch sensors33-35. Shown on the drawing is an X axis 48 and a Y axis 50. The spinner21 is centered at the intersection of the axes 48 and 50. Sensor 34 lieson the X axis 50, sensor 33 to the left thereof and sensor 35 to theright thereof.

As can be seen from the cross section of FIG. 3, which is taken throughthe left vacuum spinner 25, appropriate bores are formed in the alignerplatform 45 for the spinners. A smaller bore extends completely throughthe aligner platform with a larger bore near the top surface forming aledge 51 on which a flange 53 of the spinner can rest. In theillustrated embodiment, a bearing 55 is provided between the spinner 25and the bore. Below the flange 53 the vacuum spinners are essentiallycylindrical and contain therein a pair of circumferential grooves 57 and59 containing O-ring seals. These seal against the bore in the alignmentplatform above and below a circumferential channel 61 which is coupledthrough a channel (not shown) to a vacuum fitting 65 coupled to theassociated solenoid valve, i.e., the solenoid valve 41 of FIG. 1. Plug63 in the channel 61 restricts the vacuum to those portions of thespinner that are under the wafer being aligned. The spinner contains acentral bore 71 in which a sensor is inserted, in this case sensor 31.The sensor may be a LED-photodetector type sensor, the photodetectordetecting the reflection of the LED off the bottom of the wafer ordetecting light from a source above that is blocked by the wafer. Atransparent cover 73 is provided over the sensor 31.

The inner end of the spinner has a portion 75 of reduced diameter onwhich gear 77 is disposed. Gear 77 engages with a gear 79 on the shaft80 of a motor 81. Referring back to FIG. 1, motor 81 is driven by anoutput from the I/O port 13 on line 85 with its direction controlled byan output on line 83.

In operation, when prealigment is to take place, the wafer istransferred on air track 84, the end of which forms the platform 45, inconventional fashion, in the direction of arrow 86. Air through aplurality of holes (not shown) supports and propels the wafer. Once thewafer reaches its approximate proper position, the motor 81 will bedriven, driving the gears associated with each of the spinners in eithera clockwise or counterclockwise direction depending on whether an outputis provided on line 83 or not. Which of the spinners is activated torotate the wafer will depend upon the operation of the solenoid valves37, 39 and 41. A typical position of a wafer when first placed on theplatform is shown in dotted lines on FIG. 2. A typical sequence ofoperations which is used to align the wafer is as follows: The rightvacuum spinner rotates, for example, counterclockwise until the edge issensed at the left vacuum spinner 25 by its sensor 31. Then, the valvefor the left vacuum spinner is opened and that of the right vacuumspinner closed and the left vacuum spinner rotates clockwise untilsensor 29 senses the edge. It can be seen that now the wafer is alignedgenerally with respect to the X and Y axes 47 and 49 but it isimproperly rotated, i.e., the flat is about 90° away from where itshould be. This will cause the flat sensors to be other than equallycovered rather than equally covered as they should be. Operation of thevalve associated with the center spinner 21, and a rotation thereof,either clockwise or counterclockwise until the desired condition withrespect to sensors 33, 34 and 35 is detected, thus, takes place.

FIG. 4 is a flow diagram for the program which will be stored in themicro-processor of FIG. 1 to carry out alignment. In this program thedesignation "O" with respect to apertures means that the aperture iscompletely covered by the wafer. The designation 1 means the aperturesare not covered by the wafer. Half covered is indicated by 0.5. When thewafer is first moved by the air track on the aligner platform, uponentering the program, following start 100, a first decision is madewhether or not sensor 31 associated with spinner 25 is covered. If it isnot, the program exits to decision block 103 where it is determinedwhether or not sensor 29 associated with spinner 23 is fully covered. Ifit is also not covered, the program loops back to decison block 101. Asthe wafer moves from above in the direction of arrow 86 of FIG. 2, itwill first cover both sensors completely. When both the sensor 31 andthe sensor 29 become covered, block 103 is exited to decision block 105to see if sensor 31 is half covered. If not, then it exited to block 107to check to see if sensor 29 is half covered. If it is not, it loopsback to block 105. As the wafer moves in the direction of arrow 86 oversensor 29 and sensor 31, the program continues looping back and checkingto see if sensor 29 or sensor 31 become half covered. When one doesbecome half covered, assume it is sensor 31, decision block 105 isexited to block 109 which causes the vacuum to be applied by turning onsolenoid valve 41. Decision block 111 is entered to see of sensor 29 ishalf covered. If not, as indicated by block 121, spinner 25 is rotatedin the clockwise direction. Again, the program loops around continuingto check and once the condition is met exits to block 123 which stopsthe spinner 25.

Returing now to decision block 107, if sensor 29, is first halfuncovered, in block 125, vacuum for solenoid 39 associated with spinner23 is turned on. Decision block 127 is entered where a check is made tosee if sensor 31 is half covered. If not, the motor is driven so as todrive spinner 23 counterclockwise as indicated by block 129. Again,continual checks are made and, when the condition is met, decision block127 is exited and spinner 23 stopped as indicated by block 131.

Upon exiting block 123 or block 131, the condition of the wafer is suchthat its edges are at the center of the indicators 29 and 31. However,in the example shown in FIG. 2 the flat would be rotated approximately90° from its proper position. The situation would then be that sensors33, 34 and 35 would be partially but unequally covered because of theradius of the wafer. A decision block 163 is now entered to see if allthree are equal. In the unusual case that the flat was properly aligned,the answer would be "Yes." Exiting from block 163 the center spinner 21is stopped by stopping motor rotation. Next the decision block 165 isentered where half coverage of sensor 31 is checked. In this raresituation discussed, the answer from here would be "Yes" and a block 167is entered which directs the stoppage of spinner 23 by stopping themotor. This is followed by a decision block 169 where half coverage ofsensor 29 is checked. Again in this example, it is half covered and anexit to a block 171 which stops spinner 25 results. From here, oneenters the decision block 173 where the equality of sensors 33, 34 and35 is again checked. Again, this being "Yes", the program enters a block175 which directs stopping spinner 21 followed by the exit from theprogram in block 177, prealignment having been complete.

The more typical situation is one where the flat is not aligned afterthe first alignment of the edges. Thus, the answer in block 163 will be"No.". This will cause the vacuum to be turned on by turning on thesolenoid 37 as indicated in block 179, vacuum to the other two spinnersto be turned off, as indicated in blocks 181 and 183, and rotation ofthe center spinner 21 clockwise as indicated by the block 185. Rotationcontinues until the condition set in block 163 is met. The program thenexits to block 164 described above and into block 165. Now, quitepossibly, because of the rotation, the edges are no longer properlyaligned. Thus, when a check is made to see if sensor 31 is half coveredthe answer is "No." Solenoid 39 is energized to supply the vacuum asindicated by block 187, vacuum turned off to the other two spinners asindicated by blocks 188 and 189 and rotation on the spinner 23 clockwiseinitiated as indicated by block 190. When a "Yes" answer is obtainedfrom block 165 it is exited through block 167 and decision block 169entered. Again, assuming that the sensor 29 is not half covered, oneexits from this block into a block 191 which indicates that the vacuumshould be turned on by turning on the solenoid valve 41. Again, vacuumis turned off to the other two spinners as indicated by blocks 192 and193 followed by a counterclockwise rotation of spinner 25 as indicatedby block 194. Once this condition is met, block 169 is exited throughblock 171 and block 173 entered. Because of the adjustment of the edges,the condition set by block 173, which was the condition of block 163 mayno longer be true. If it is not, then, as indicated by block 195,solenoid 37 is closed to supply vacuum to spinner 21 and vacuum turnedoff to the other two spinners as indicated by blocks 197 and 199. Acheck is then made in decision block 201 to see whether the left or theright sensor, i.e., sensor 33 or 35 is greater. If more of sensor 35 isuncovered then a clockwise rotation is necessary. Conversely, if more ofsensor 33 is uncovered counterclockwise rotation is necessary. This isindicated by the respective block 203 and 205 the outputs of which joinat the input to decision block 207 which again checks for the equalityof the three. As long as the answer is "No" from this block, it loopsback to the input of block 201 and rotations continue until the desiredcondition is met. Then block 207 is exited to block 175 explained above,the motor stopped and prealignment completed. It should be exident thatthis additional step of aligning the flat could again throw off thealignment of the edges. Thus, it would be possible upon exiting block207 to go back to block 165 and go through another iteration. Prior tocarrying out rotation of block 190 or of block 194 for this iteration itis desirable to check to see whether or not the respective sensor ismore than half covered of less than half covered. If such a check ismade then the direction of rotation can be selected to provide theshortest distance of rotation. As many iterations as necessary may becarried out to give the desired degree of alignment accuracy.

The disclosed arrangement of spinners and sensors has been found to beparticularly advantageous and efficient. However, the invention can beimplemented with other arrangements of on-axis and off-axis spinnersalong with other sensor arrangements and logic schemes. In general, atleast two off-axis spinners will be required along with one on-axisspinner. The off-axis spinners along with the edge sensors are utilizedto get the wafer properly aligned with respect to the X and Y axes. Thecenter spinner is used basically to then rotate the wafer so that itsflat or notch is properly aligned.

What is claimed is:
 1. An apparatus for prealinging a circular wafercomprising;(a) an aligner platform having defined thereon X and Y axesand a standardized reference X, Y and rotational orientation for awafer; said wafer having a flat or notch formed in the peripherythereof, (b) a first vacuum spinner, first means supporting said vacuumspinner for rotation in said platform about the intersection of the Xand Y axes; (c) at least second and third spinners, second meanssupporting said second and third spinners for rotation in said platform,displaced from said intersection of said axes; (d) at least two edgesensors for sensing the required wafer edge location at two pointsseparated from each other; (e) at least one flat or notched sensor forsensing rotational alignment of said wafer; and (f) logic means forreceiving inputs from said sensors and providing outputs to said vacuumspinners for carrying out a series of individual rotations to align saidwafer.
 2. Apparatus according to claim 1, wherein second means rotatesaid second and third off-axis vacuum spinners about points located onthe peripherey of the wafer when in the required position and whereinsaid first and second edge sensors are disposed at the centers of saidsecond and third vacuum spinners.
 3. Apparatus according to claim 2,wherein said second and third vacuum spinners are equally spaced fromone axis of said alignment platform.
 4. Apparatus according to claim 3,wherein said flat or notch sensors comprise three sensors disposed in aline corresponding to a line of said flat or notch, one sensor on anaxis to which said notch or flat is to be perpendicular and the othertwo sensors equally spaced therefrom.
 5. Apparatus according to claim 1or 4, wherein said logic means comprise a programmed micro-processor andassociated input output ports.
 6. A method of prealigning a wafer on aprealignment platform having a central axis comprising:(a) linearlymoving said wafer along a central axis of said platform, (b) sensingsaid wafer at a pair of positions equally spaced from said central axis,(c) spinning said wafer in a first direction until the edge of saidwafer is detected at one of said positions, (d) spinning said wafer in asecond direction until the edge of said wafer is detected at the otherof said positions, (e) stopping said wafer from spinning when the edgeof said wafer is sensed simultaneously at said pair of positions.
 7. Amethod according to claim 6 further including;rotating said wafer afterthe edge has been sensed simultaneously at said pair of positions,sensing a notch or flat on said wafer, stopping the rotation of saidwafer when said notch of flat has been sensed.
 8. A method according toclaim 7 further including;performing the steps of claim 6 after therotation of said wafer is stopped as result of a flat or notch.